Modification of old HPS piping 5

[deepu78]

A modification is suggested by operations to modify a High Pressure Steam piping to install a new type inline type desuperheater.
Operating pressure 870 PSIG and Temperature 950 degree F.The plant is 20 years old.
To do this modification,what are the critical things to consider?
DO we need to consider existing corrosion(wall thickness) in the line? Can stress analysis identify whether flange joints can be introduced in this line?
What are the codes to be considered in this modification?This is a stress relived line,so PWHT needed after welding?There is a suggestion of reducing the line diameter for a 3 meters for the effective working of desuperheater.
How all these to be considered in evaluation and planning stage?

DV

 

[mk3223]

During the preliminary project planning stage, first to consult the process for the proper operation process data as needed for the desuperheater; contact the vendor for a equipment proposal; and have the piping engineer to estimate the manhour of the piping work needed, including material design and specification, piping modification and stress evaluation, etc.

 

[1503-44]

Deep, you did not say what modification is being proposed. Without other info being supplied to the contrary, reducing anything like pipe diameter implies you are planning on downsizing capacity.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Dhurjati Sen]

1. Replace with identical piping, corrosion is probably negligible as the existing has served for over two decades.
2. Surely design team needs to carry out necessary calculations for installing flanges.
3. The new piping should be as per the existing code.
4. The piping code would tell you if PWHT is required.
5. Again, the design team needs to look into changing the new piping dimensions.

DHURJATI SEN
Kolkata, India

 

[1503-44]

Famous last words? "corrosion is probably negligible as the existing has served for over two decades." I think that would be the first thing to check.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[deepu78]

The modification is a Spray type desuperheater(STD) which is inline to the High pressure steam line (instead of existing VAD desuperheater)Data sheet attached.
1.For the effective working of the desuperheater the line has to be of smaller size just before the STD(80NB instead of current 150NB) for about 3 meters,then back to the original size 150NB. How to go about this change if process and vendor agreed to this solution?
Do the line number to be changed?
Whether pipe design and stress analysis will be able to analyze whether this modification will pose a problem during operation?

 

[deepu78]

The existing VAD data sheet attached.

 

[1503-44]

Sounds and looks like somebody must actually do some system verification and a flow and stress analysis to answer your questions.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Snickster]

DO we need to consider existing corrosion(wall thickness) in the line? Can stress analysis identify whether flange joints can be introduced in this line?

I would base any calculations based on the original pipe wall thickness minus corrosion allowances. I would check for actual corrosion when the pipe is cut open. If corrosion allowance has been used up then would need to replace piping. If significant corrosion allowance still exists then piping is not corroding and no worries. If significant corrosion is present but still have some corrosion allowance remaining I would install corrosion coupons to monitor future corrosion. A stress analysis is not absolutely required in this instance but recommended - see below. A stress analysis is not required to determine if flanges can be added to the pipe to add the flanges required for the STD. A stress analysis does not analyse the stresses in the flanges but only considers flanges as rigid bodies. Flanges are just added in the piping as required. However a stress analysis has the abilities to check if the flanges will leak by performing an analyis to check if the bending and pressure stresses will exceed limits and cause flanges to leak.

What are the codes to be considered in this modification?

Either ASME B31.1 or B31.3 depending on which is more applicable or what the plant was originally designed to.

This is a stress relived line,so PWHT needed after welding?

Whatever the applicable code states.

There is a suggestion of reducing the line diameter for a 3 meters for the effective working of desuperheater.

From what I understand you will removing an existing VAD desuperheater with at least 6 meters of straight pipe downsream. If you replace with the STD spray desuperheater you will only be cutting out the existing desuperhater and some straight pipe and replacing it with the STD superheater with flanges and some smaller diameter 3 meters of straight pipe upstream. Since you are not adding any bends to the line there should be no change in the stress in the pipe because you are not changing the basic configuration - it is just a straight pipe run just like before the change. It does not matter stress-wise if you change the diameter for 3 meters. Basically you really should be OK without any analysis since stress-wise you are not changing anything but I would still check with a sress calculation program such as Caesar up to a point in the existing piping where any effect of any changes will not be felt by the existing piping - such as back to an exisiting anchor or line stop. In this way you could verify that the flanges you added will not be overloaded and leak.

1.For the effective working of the desuperheater the line has to be of smaller size just before the STD(80NB instead of current 150NB) for about 3 meters,then back to the original size 150NB. How to go about this change if process and vendor agreed to this solution?

See replies above

Do the line number to be changed?

If you are not changing the pressure rating of any section of the line there is no need to change line numbers.

Whether pipe design and stress analysis will be able to analyze whether this modification will pose a problem during operation?

See responses above

 

[KevinNZ]

Do not change pipe to smaller size without stress analysis. The smaller pipe will attract load/stress from the larger/stiffer pipe either side. You have no idea of the stress in the original large pipe and the smaller pipe will have higher stress.

I would also recommend stress analysis for adding flanges. The extra weight will add to EQ and gravity loads with the piping and on supports.

 

[MJCronin]

My two cents ....

1) Downstream piping droplet erosion is a phenom not commonly considered in the planning stage. It can be significant in later years as the plant ages and especially if the system is subject to lots of startup-shutdown cycles. I would discuss this erosion issue with the DS vendor ... especially since your piping is ~20 years old

2) Noise is another phenom not often considered. Will your DS generate lots of noise ?...Better to find out now !!!

3) Similar posting on eng-tips:

https://www.eng-tips.com/viewthread.cfm?qid=432085

Deep, do not drop this thread as you have so many times in the past .... Please post pictures of your installation and tell us about the design challenges that you faced

Please complete this thread, sir ..

Best Regards and good luck ...

MJCronin
Sr. Process Engineer

 

[georgeverghese]

As always, the process engineer's requirement for flow turndown will influence desuperheater selection / configuration.